Turbine bucket cover

ABSTRACT

A damping-type shroud covering for turbine buckets wherein one embodiment consists of a double layer of overlapping cover bands each spanning two adjacent buckets. The inner bands are loosely disposed between buckets whereas the outer layer rigidly joins two buckets. Another embodiment consists of an over and under cover band wherein each cover band is tightly joined to one bucket and loosely joined to the adjacent one.

United States Patent lnventors Victor S. Musick Scotia; Joseph H. Ouellette, Albany, NY. Appl. No. 815,450 Filed Apr. 11, 1969 Patented Mar. 30, 1971 Assignee General Electric Company TURBINE BUCKET COVER 7 Claims, 6 Drawing Figs.

US. Cl 416/190, 416/191 Int. Cl F01d 5/22 Field 01 Search 253/77 (5- References Cited UNITED STATES PATENTS 1,129,920 3/1915 Thompson 416/191 1,842,957 1/1932 Bassler 416/190 2,221,678 11/1940 Hecknam 416/190 3,048,365 8/1962 Foster et a1. 416/190 3,367,629 2/1968 Partington 416/190 FOREIGN PATENTS 377,101 7/1932 Great Britain 416/195 Primary Examiner-Everette A. Powell, Jr Attorneys-William C. Crutcher, Frank L Neuhauser and Oscar B. Waddell Pgientel Mm]. 30, 1971 2 Sheets-Sheet l INVENTORS VICTOR S. MUSICK, JOSEPH H.0UELLETTE,

THEIR ATTORNEY.

Patented March 30, 1971 3,512,968

2 Sheets-Sheet z INVENTORSZ VICTOR S. MUSICK, JOSEPH H. OUELLETTE,

THEIR ATTORNEY.

TURBINE BUCKET COVER BACKGROUND This invention relates generally to turbomachinery rotors and more particularly to an improved shroud construction for rotor buckets.

Adjacent rotor blades or buckets are normally connected together by some form of cover bands or shroud bands around their outer periphery to confine the working fluid within a well defined path, so as not to spill outwardly past the ends of the buckets, and also to increase the rigidity of the buckets. Generally, the cover bands connect groups of buckets together, a reason being that the banding of buckets into groups distributes any stimulus present in the working fluid over a group of buckets and thus decreases the stimulus acting on a single bucket. One unfavorable effect of this group banding is that the cover itself can be stimulated in a resonant manner by a number of impulse frequencies corresponding to different natural vibration modes of the bucket-cover combination.

A large number of buckets per cover group is desirable from the standpoint of reducing the stimulus per bucket, whereas a small number of buckets per cover group is desirable to keep the number of vibrations which can be excited in a single cover segment at a minimum and also to keep the vibration spectrum at a high frequency.

Efforts to solve these problems have manifested themselves in various forms. One early structure utilized two cover bands that were separated by a loose radial clearance developed by a piece of mica that was employed to separate the two covers as the tenons were peened over. This was somewhat inefficient. Another early form had two cover bands but both were rigidly connected to the bucket tips with the outer cover spanning four buckets thus losing the advantage of a small number of buckets per cover band. A still further and more recent method used to solve the aforementioned problems utilized two cover bands but both of which were rigidly connected to the bucket tips with no radial clearance therebetween. This design also suffers in that it does not produce a small number of buckets per cover band.

Accordingly, it is an object of the present invention to provide a shroud construction that distributes a stimulus over a group of buckets.

Another object is to provide a shroud construction whereby the number of vibration frequencies which can be excited in a cover segment is kept at a minimum, and also one in which the vibration spectrum is maintained at a high frequency.

SUMMARY OF THE INVENTION Briefly stated, the present invention is practiced by a turbomachine bucket and shroud assembly wherein a tenon extends from the tip of each bucket and is provided with a step. An outer cover band portion is rigidly connected to the top of the tenon step while an inner cover band portion is loosely connected to the bottom of the tenon step. The cover band portions overlap one another at the tenon to provide efficient damping and at least one of the cover band portions extends to the adjacent bucket and is rigidly connected to the top of its tenon step. Upon assembly the top of each tenon step is peened over to provide the necessary rigidity.

DRAWING The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of practice, together with further objects and advantages thereof, may best be understood by reference to the following description, taken in connection with the accompanying drawing in which:

FIG. l is an elevation showing the shroud covering in one embodiment of this invention;

FIG. 2 is a plan view showing the overlapping shroud covering in the same embodiment as in FIG. 1;

FIG. 3 is an exploded perspective view showing the detail of a bucket tip and the cover bands;

FIG. 4 is an elevation showing the shroud covering in an alternate embodiment of this invention;

FIG. 5 is a plan view showing the overlapping shroud covering in the same embodiment as in FIG. A; and

FIG. 6 is an exploded perspective view showing the detail of a bucket tip and the cover bands in the alternate embodiment.

DESCRIPTION Referring now to FIGS. 1-3, a typical environment of this invention is shown including a plurality of buckets 2 which extend radially from a rotor (not shown). Extending radially rom the tips of buckets 2 are tenons 4 consisting of a step 5 which divides the tenon 4 into a base portion 6 and a top portion 7. This configuration of tenon 4 is provided so as to maintain proper clearances and will be more fully described in a later part of this description.

Extending circumferentially around the tips of buckets 2 and disposed over tenons 4 are two layers of cover or shroud bands. The inner cover or shroud generally indicated as 8, is comprised of inner cover bands 9, each. of which spans two adjacent buckets. Each inner cover band is disposed over a tenon base 6 and is constructed in such a manner as to provide circumferential clearaiice 10 between the tenon base and inner cover band. A radial clearance 11 is provided between buckets 2 and inner cover bands 9 due to the configuration of tenon step 5 and thickness of cover band 9. A circumferential clearance 12 is maintained between adjacent inner cover bands 9 so as to prevent a rigid coupling between buckets 2.

Outer cover bands 13 are constructed similarly to cover bands 9 except that there is only a small clearance for assembly purposes defined between each tenon top 7 and each outer cover band 13, that is, a substantially rigid connection between each cover band 13 and its associated tenon 4 is maintained. Upon assembly the top of tenon 4 is peened over so as to increase the rigidity of the coupling and to hold the covers on. A circumferential clearance i3a is defined between adjacent outer cover bands 13. Outer cover bands 13 are staggered with respect to the inner cover bands during assembly so that they overlap the inner cover bands 9.

Referring now to FIGS. 4-6, a description of an alternate embodiment of the present invention will be given, wherein like elements of the first embodiment are represented by the same reference numbers. A continuous: segmented cover band is provided consisting of over and under cover bands 14. Each over and under cover band l4 extends from the tenon top 7 of one bucket to the tenon base it of the adjacent bucket. As in the first embodiment, an outer cover band portion is disposed over the tenon top portion 7 in a substantially rigid manner while the inner cover band portion of each over and under segmented cover band 14 is loosely disposed over the tenon base portion 6 of an adjacent bucket. At each tenon 4 an outer cover band portion overlaps an inner cover band portion with the outer cover band portion extending to the adjacent bucket through a curvilinear portion 15 where it becomes an inner cover band portion which defines a clearance ll between the buckets 2 and cover bands M. Also, as in the embodiment of FIGS. 1-3, a tenon clearance 10 is provided between tenon base 6 and cover bands M.

OPERATION The operation of the present invention will now be described. in FIGS. l-3 it will be appreciated that each outer cover band in effect rigidly connects the two adjacent buckets, while each inner cover band in effect loosely connects two ad jacent buckets with the outer cover bands overlapping the inner cover bands. As the turbine is brought up to speed, the inner cover bands are forced radially outward due to the centrifugal force of rotating machinery. At speed, each inner cover band together with each outer coverband fonns a semirigid bucket shroud which will, in effect, distribute any stimulus present in the working fluid flow over a large bucket group. At the same time, the inner and outer cover bands are not so rigidly joined that the cover itself will be stimulated in a resonant manner by a number of impulse frequencies corresponding to different natural vibration modes of the bucketcover combination. Frictional forces between bucket covers help to damp out unwanted vibrations.

The operation of the embodiment in FIGS. 4-6 is similar to the embodiment as shown in FIGS. I3 and just described except that no two buckets are rigidly joined together. As the rotor is brought up to speed centrifugal force acting on the "over and under cover bands will tend to move the inner cover band portion outward toward the bottom of the next outer cover band portion due to the radial clearance 11 provided, at which point frictional and the aforementioned centrifugal force will cause the inner and outer cover band portions disposed on each tenon to form a dynamically rigid shroud. it will be appreciated that a small number of buckets per group is effectively maintained since the inner cover band portion is not connected in a stationary manner while at the same time a large number of buckets per group is effectively maintained because the cover bands are in physical contact when at speed thus allowing the distribution of any stimulus present.

it will be appreciated that two embodiments of a bucket shroud'assembly have herein been described which achieve the advantages of both a large and a small number of buckets per cover group. Rigid coupling between bucket groups is prevented but effective coupling is accomplished at speed by centrifugal and frictional forces.

It may occur to others of ordinary skill in the art to make modifications of this invention which will remain within the concept and scope thereof and will not constitute a departure therefrom. Accordingly, it is intended that the invention be not limited by the details in which it has been described but that it encompass all within the purview of the following claims.

We claim:

1. ln a turbomachine having at least one stage of radially extending buckets and a shroud assembly extending circumferentially around the tips of said buckets, the combination comprising:

at least one tenon extending radially from the tip of each of said buckets and having a step with a base portion and a top portion;

an outer cover band portion having at least one hole therein, rigidly connected to said tenon top portion;

an inner cover band portion having at least one hole therein,

loosely connected to said tenon base portion;

said cover band portions overlapping one another at the tenon to provide damping and at least one of said cover band portions spanning two adjacent buckets and rigidly connected to said tenon top portion of an adjacent bucket.

2. A turbomachine bucket and shroud assembly according to claim 1 wherein the tenon top portion is peened over to provide a rigid connection.

3. A turbomachine bucket and shroud assembly according to claim 1 wherein a radial clearance is defined between said inner cover band portion and said bucket tip and also a circumferential clearance between said tenon base portion and said inner cover band hole.

4. A turbomachine bucket and shroud assembly according to claim l wherein said outer cover band portion is rigidly connected to the tenon top portions of two adjacent buckets while said inner cover band portion is loosely connected between the tenon base portions of two adjacent and staggered buckets.

5. A turbomachine bucket and shroud assembly according to claim 4 wherein a clearance space is defined between the ends of said inner and outer cover bands.

'6. A turbomachine bucket and shroud assembly according to claim 1 wherein a cover band segment spans eve two adacent buckets and overlaps an ad acent segment at e tenon to provide said inner and outer cover band portions, one end of the segment being rigidly connected to said tenon top portion of one bucket, the other end being loosely disposed over said tenon base portion of said adjacent bucket.

7. A turbomachine bucket and shroud assembly according to claim 6 wherein said cover band segment has a curvilinear portion in order to make the transition from said tenon top portion to said adjacent tenon base portion. 

1. In a turbomachine having at least one stage of radially extending buckets and a shroud assembly extending circumferentially around the tips of said buckets, the combination comprising: at least one tenon extending radially from the tip of each of said buckets and having a step with a base portion and a top portion; an outer cover band portion having at least one hole therein, rigidly connected to said tenon top portion; an inner cover band portion having at least one hole therein, loosely connected to said tenon base portion; said cover band portions overlapping one another at the tenon to provide damping and at least one of said cover band portions spanning two adjacent buckets and rigidly connected to said tenon top portion of an adjacent bucket.
 2. A turbomachine bucket and shroud assembly according to claim 1 wherein the tenon top portion is peened over to provide a rigid connection.
 3. A turbomachine bucket and shroud assembly accordiNg to claim 1 wherein a radial clearance is defined between said inner cover band portion and said bucket tip and also a circumferential clearance between said tenon base portion and said inner cover band hole.
 4. A turbomachine bucket and shroud assembly according to claim 1 wherein said outer cover band portion is rigidly connected to the tenon top portions of two adjacent buckets while said inner cover band portion is loosely connected between the tenon base portions of two adjacent and staggered buckets.
 5. A turbomachine bucket and shroud assembly according to claim 4 wherein a clearance space is defined between the ends of said inner and outer cover bands.
 6. A turbomachine bucket and shroud assembly according to claim 1 wherein a cover band segment spans every two adjacent buckets and overlaps an adjacent segment at the tenon to provide said inner and outer cover band portions, one end of the segment being rigidly connected to said tenon top portion of one bucket, the other end being loosely disposed over said tenon base portion of said adjacent bucket.
 7. A turbomachine bucket and shroud assembly according to claim 6 wherein said cover band segment has a curvilinear portion in order to make the transition from said tenon top portion to said adjacent tenon base portion. 